From my perspective answers are:

1) Yes I would put down the plywood especially if your going to do solar panels. But make certain the roof decking that is there is sound because if it is rotten or "punky" you will just be exposing the new plywood to the same conditions and be making matters worse because that will rot as well.

2) Foam is on R-7 per inch so using foam will help but you will never get anywhere near R-38.

3) If you do not have a detailed proposal without asking for one find another roofer.

when was the last time it was roofed? How many layers are up there now? If you're going down to the deck then all punky/rotted wood should be replaced. You don't say where you are but if you need R-38 then you're likely in a colder high-snow area.

Make sure that you get ice-water shield to prevent water intrusion from ice dams.

Ventilate the roof and insulate the attic. Conventional asphalt single roofs do not have exterior insulation.

the proposal should include all of the details fo the work. If it's not comprehensive then I wouldn't dismiss the roofer immediately but make sure get acceptable answers to all concerns before signing/commissioning the work.

The actual condition of the roof sheathing can only be determined by actually looking at/testing the wood. If the previous roofing was done correctly, that decking could be in good shape. If the attic was improperly vented, the decking could be moldy/degraded.

That can be done on about 80% of the sheathing by inspecting the underside from inside the attic. That means an extra cost---if a roofer will even do that.

A competent roofer will be able to quote the cost of plywood installation for the entire roof, including the tearoff costs.

They should give you the option of which shingle to use, the color, and type(single layer or architectural) along with the cost of installation.

That amount will also cover the type of vents and the cost for those.

If there are soffits with vents built in, a ridge vent would be the best option. Totally passive and fool proof as long as the soffit vents are open.

Now is also the time to change any venting(kitchen/bath/plumbing) that is not run through the roof and terminated inside the attic to properly vented through the roof.

Thanks for the advice, everyone!

Millworkman, I'm leaning toward putting down the plywood. I will definitely check out the decking.

Sdello, I'm in the Boston area. I don't know how old my roof is, but it's probably 20 years or older. I've been told that it had a few years left, but a tree branch fell on it and tipped my hand. The insurance won't cover a whole new roof, but it's better to go ahead and do the whole roof because a repair wouldn't match, and the roof is nearing the end of its life.

Handymac, I believe there's only one layer of shingles right now, and I'll definitely have them torn off. (Obviously, if I put down plywood, leaving them wouldn't be an option anyway.) I think the roof is in pretty good shape (apart from the tree damage), just old. Because of the foam insulation under the roof, it's impossible to tell from inside what shape the decking is. I'll have to trust the roofer to make the right call.

All my plumbing is vented through the roof, so I'm OK on that count.

You'll want your contract to mention that all work will be done to a minimum of code.

You'll want new decking, specify if it'll be OSB-type or plywood, and specify the thickness to be installed.

That ice/water shield will be installed as appropriate.

What type of paper? 15-lb felt? One of the proprietary papers?

That all flashing will be replaced. You can leave that vague, or you can get specific; material, color, specific type.

Specify drip edge if you want it on rake and eave edges.

Is there a chimney that will need to be flashed? Specify flashing and counter-flashing that will allow for movement versus the roofer using a one-piece.

New boots to be installed over plumbing stacks.

The contract should mention the specific type and color of shingles to be used. The original proposal might not since you haven't picked out a shingle.

Is the roof vented or not vented? If it is, you'll want to address that.

With the tear off, the contract should also mention debris removal and cleanup.

I'm in a coastal area, we need high-wind nailing schedules, etc. That may or may not apply to you.

You can even go so far as to discuss what types of nails will be used; hot-dipped galvanized, etc.

If you haven't done it yet, simply peruse a shingle manufacturer's website. Photos and "how to" info might lead you towards feeling more comfortable about your project.

Thanks, Mongo. That degree of detail is very helpful. Just for the record, I'm in Massachusetts, about 25 miles inland. Not a high wind area, but we do get the occasional hurricane.

Now I have a couple of follow-on questions (for you or anyone else with an opinion).

1) OSB vs plywood - What are the pros and cons? I understand that OSB would be cheaper. Any other important differences? Which would hold nails better or hold up longer?

2) 15-lb felt paper vs proprietary paper? I wouldn't expect the manufacturers' sites would be particularly objective on this one.

Thanks!

P.S. Sorry not to reply sooner, but for some reason I was having problems reaching this site.

First check and find out if a roofer has a license; bond and insurance and ask to get copies of the documents. Most roofing manufactures can assist in selecting a reputable contractor as they know who they supply and how long they have been in business. Check Angie's List or the BBB and get their ratings and reviews. Ask questions like do they use starter strips(required for many mfg. warranties); weight of felt used (15# or 30# depending on pitch); what about drip edge and decking; ridgevent included (if you do not have ridgevent make them provide it); and ask for and check references. Do not ever sign anything until you are comforatble with whom you are dealing with! Your insurance agent and some adjusters can also give you feedback. And if they say they need "upfront" or "seed" money beware, they may not have good credit with their suppliers. Most have 30 day terms and if they have been around awhile they do not need money until either the materials drop or the work is complete. Make them let you choose the brand shngle to be used, it's your home and Consumer Reports can tell you there is a difference in quality. There are some very reputable and quality roofers out there but a lot of goofballs. Do your homework and work with the guy you feel will do what he/she says they will do and do it up to code and standards. Check to see how long has the company been in business and are they local? Some people travel the country chasing storms. If you have an issue they won't be there to do warranty work! Our state (SC) requires a two year guarantee on workmanship, we give 5 years. Finally go to manuacturer's websites and look at warrantees, colors, etc. There is a lot of information on the internet put out by manufacturer's who can answer questions and help! They like a good roofer want you happy!